The present invention relates to a pupil modulation optical system and, more particularly, to a pupil modulation optical system used in the optical system of a microscope or the like to improve the image quality by increasing contrast and improving resolution.
As devices which are similar to the pupil modulation optical system of the present invention, ordinary circular obscured aperture and apodization filters have heretofore been known: for example, see Japanese Patent Application Laid-Open (KOKAI) No. 62-212617 (1987) and Japanese Patent Application Post-Exam Publication No. 03-81129 (1991).
In any of the above-described ordinary circular obscured aperture and apodization filters, pupil modulation is effected at only one position (a pupil position of an objective or condenser lens) in the optical system. Therefore, the effect of pupil modulation is fixed, and various pupil modulation effects cannot readily be obtained. That is, it is not easy to change over observation modes, i.e., observation with high contrast, and observation with high resolution, in accordance with each particular specimen. In Japanese Patent Application Laid-Open (KOKAI) No. 62-212617 (1987) also, pupil modulation is effected at only one position, and the effect thereof is manifest only when the intensity distribution of incident light is a Gaussian distribution. In Japanese Patent Application Post-Exam Publication No. 03-81129 (1991), a circular obscured aperture is provided at a pupil position of a condenser lens, and intensity modulation and phase modulation are effected at a pupil position of an objective lens, thereby realizing high resolution and high contrast. However, the production of a phase modulation filter is difficult, and various pupil modulation effects cannot readily be changed over with the arrangement described above.